Lunar Laser Ranging - A Comprehensive Probe of Post-Newtonian Gravity

Using more than 30 years of lunar laser ranging (LLR) data, the lunar orbit has been modeled and fit with several millimeters precision. This fit provides comprehensive confirmation of the general relativistic equations of motion for solar system dynamics, achieving several key tests of Einstein’s tensor theory of gravity, and strongly constraining presence of any additional long range interactions between bodies. Earth and Moon are found to fall toward the Sun at rates equal to better than a couple parts in 10, confirming both the universal coupling of gravity to matter’s stress-energy tensor, and gravity’s specific non-linear coupling to itself. The expected deSitter precession (with respect to the distant ’fixed’ stars) of the local inertial frame moving with the Earth-Moon system through the Sun’s gravity is confirmed to 3.5 parts in 10 precision (± .07 mas/year), and Newton’s gravitational “constant” indeed shows no cosmological time variation at the part in 10 per year level. Most all of the 1/c order, post-Newtonian terms in the N-body equations of motion — motional, gravitomagnetic, non-linear, inductive, etc. — contribute to the measured details of the lunar orbit, so LLR achieves near-completeness as a gravity experiment and probe. The precision of these measurements, especially those connected with lunar orbit frequencies and rates of change of frequencies, will further improve as LLR observations continue into the future with use of latest technologies.